You can type ‘20mm’ for instance, as a type or baseline grid size in InDesign, and it will automatically convert it to points. Still, it should be a feature, especially as it already exists in the otherwise typographically inferior Illustrator.
As someone who’d happily decimalise time, I can sympathise, if not quite agree, with your stance on the abolition of imperial measurements!
I feared that answer.
I know I can type in 20 mm and InDesign automatically converts it to points. But unfortunately the measurements will still be displayed in points, which is very inconvenient if you want to calculate sizes quickly. So if I type in 20 mm it will always be displayed as 56.7 points. And there is no way to change that.
Even by scripting you cannot do this.
Though there are two promising properties for this in the scripting DOM (Document Object Model), it cannot be done.
There are only "Points" allowed. Other measurement units return an error.
Look up "textSizeMeasurementUnits" and "typographicMeasurementUnits" here: